Neurotransmitters and other neuromodulators are derivatized as functionalized congeners for the purpose of receptor studies and designing new, therapeutic agents. The functionalized congeners are designed to be potentially reactive towards drug carriers, molecular probes for the receptor, or solid supports. Functionalized congeners of adenosine receptor agonists and antagonists (di-alkylxanthines) have been synthesized. The congeners bear an amino or a carboxylic acid group and can be linked to carriers to form conjugates of high affinity for the receptor.